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Wave Runup on Atoll Reefs

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Wave Runup on Atoll Reefs MSc. Thesis Wave runup on atoll reefs Ellen Quataert January 2015 Front cover: Aerial view of the southern tip of the Kwajalein Atoll in the Republic of the Marshall Islands. Source: www.fayeandsteve.com Wave runup on atoll reefs Ellen Quataert 1210070-000 © Deltares, 2015, B Keywords Runup, atoll reef, XBeach, infragravity wave, wave-induced setup, incident swash, infragravity swash, Kwajalein Summary The aim of this research was to take the first step in understanding the wave runup process on an atoll reef using the XBeach model. Field data collected from 3 November 2013 to 13 April 2014 at Kwajalein Atoll in the Republic of the Marshall Islands was used. The dataset included data on bathymetry, waves, water levels and wave-induced runup. The data was analysed and subsequently used to model the hydrodynamics across the reef and the wave runup. The hydrostatic and non-hydrostatic XBeach models were used to capture both components of runup, infragravity and incident swash. Finally, a conceptual model was created to investigate the effect of variations in the atoll reef parameter space on the wave runup. References 1210070-000 Version Date Author Initials Review Initials Approval Initials Jan. 2015 E. Quataert A.R. van Dongeren F. Hoozemans A.A. van Rooijen State final Wave runup on atoll reefs Wave runup on atoll reefs by Ellen Quataert in partial fulfilment of the requirements for the degree of Master of Science in Civil Engineering Delft University of Technology January 2015 In collaboration with: Graduation committee: Prof. dr. ir. M.J.F. Stive Delft University of Technology ir. A.A. van Rooijen Deltares / Delft University of Technology dr. C.D. Storlazzi USGS-Pacific Coastal and Marine Science Center dr. ir. A.R. van Dongeren Deltares dr. ir. M. Zijlema Delft University of Technology Wave runup on atoll reefs Abstract Coral reefs are capable of protecting mainland and island coasts from the impacts of severe storm waves and storm surges, the impact of tsunamis, and help limit coastal erosion. Despite the protective capabilities of coral reefs, coastlines fronted by coral reefs are still threatened by the ocean. For example, a series of inundation events on islands across the western Pacific (e.g. Wake Atoll, Guam and Kwajalein Atoll) in December 2008 resulted in severe damage on the islands. Many of these islands are low-lying atolls and have maximum elevations of 3 to 5 m above mean sea level. An atoll is a circular reef that, partially, encloses a lagoon which can contain a freshwater lens. Wave-induced overwash and inundation threatens infrastructure and freshwater resources, which has consequences on the suitability of the islands for agriculture and habitation. The inundation events resulted in the need for a better understanding of the processes causing these inundation events on atoll islands. An investigation on this matter is carried out by the joint U.S. Geological Survey (USGS), National Oceanographic and Atmospheric Administration (NOAA) and University of Hawaii. The primary goal of the investigation is to determine the influence of sea-level rise on wave driven inundation and the resulting impacts to infrastructure and freshwater resources on atoll islets in the Pacific and Indian Oceans. Coasts fronted by (coral) reefs have some important physical differences with sandy coasts. The steep fore reef, the relatively long and shallow reef flat and the higher bottom roughness enforces specific hydrodynamic processes. The resulting water level variations, wave propagation and forcing across the reef and the wave-induced runup are the main processes driving inundation events. The majority of the studies that have been conducted have focused on the hydrodynamics across reefs. Little is known about the wave-induced runup in the presence of a reef. Therefore, the aim of this thesis was to take the first step in understanding the wave runup process on an atoll reef using the XBeach model. In this study, field data collected from 3 November 2013 to 13 April 2014 at Kwajalein Atoll in the Republic of the Marshall Islands was used. The dataset included measurements on bathymetry, waves, water levels and wave-induced runup. The data was analysed and subsequently used to model the hydrodynamics across the reef and the wave runup. The hydrostatic and non-hydrostatic XBeach models were used to capture both components of runup, infragravity and incident swash. Finally, a conceptual model was generated to investigate the effect of variations in the atoll reef parameter space on the wave runup. The data was analysed to derive the wave heights, wave-induced setup and wave runup along the cross-shore reef profile. The wave height evolution over the reef towards the shore showed a general trend of increasing low frequency (infragravity and very low frequency) waves and decreasing high frequency (short) waves. Especially during the energetic wave events, the IG and VLF wave heights increase substantially and became dominant near the shoreline. Furthermore, it was found that on the reef flat the wave heights, wave-induced setup and wave runup are all strongly affected by the offshore tidal variations. Runup observations were collected for the period between 3 November 2013 and 17 December 2013 and included one extreme runup event on 17 November at 16:17. The extreme wave runup event on the Kwajalein Atoll is significantly underestimated by the hydrostatic XBeach model. The non-hydrostatic model includes the incident swash in the simulations and showed an improved wave runup prediction, despite the underestimated Wave runup on atoll reefs i wave-induced setup. However, still a clear underestimation of the wave runup was found, which was explained by an instantaneous increase in surface elevation in the raw burst of data at the inner reef flat pressure sensor. For the measured extreme wave runup event at Kwajalein Atoll, it is therefore not sufficient to schematize the hydraulic conditions into hourly burst-averaged characteristics for this specific event. The XBeach hydrostatic model showed good results for the hydrodynamics across the reef, but the non-hydrostatic model performed better on the wave runup. Extrapolating these results to storm conditions, it is expected the contribution of the incident swash to the total wave runup increases significantly, as the water depth on the reef flat increases. This effect was substantiated by the conceptual model simulations. Furthermore, the impact of incident swash is expected to become stronger considering the effects of rising sea levels. However, further investigation is needed on the underestimation of the wave-induced setup on the reef flat by the non-hydrostatic model. If this issue can be solved, the non-hydrostatic model is capable to model the wave runup (and subsequently inundation events) more accurately. ii Acknowledgements With this thesis the Master of Science programme at Delft University of Technology is completed. The research was carried out at Deltares in Delft and USGS-Pacific Coastal and Marine Science Center in Santa Cruz, California. I would like to greatly thank my graduation committee for their support and enthusiasm throughout my thesis. My daily supervisors at Deltares, Ap van Dongeren and Arnold van Rooijen, for their inspiring and skilled supervision and the time they invested in me. Thanks to Marcel Stive and Marcel Zijlema for their enthusiasm and guidance during the meetings we had. Special thanks to Curt Storlazzi for the guidance during my stay in Santa Cruz. Your enthusiasm for this research was very encouraging. I have learned a lot about coral reef hydrodynamics and modelling for which I thank you all. I had the opportunity to spend three months within this research at the U.S. Geological Survey in Santa Cruz, California. Many thanks to the USGS and Deltares for giving me this opportunity, it has been a unforgettable and valuable experience. Many thanks to everyone at the USGS in Santa Cruz for making me feel at home in Santa Cruz. Special thanks to Olivia Cheriton for the help and the interesting conversations we had on data analysis. Finally, and most importantly, I would like to thank my parents and sisters for the continuous support, encouragements and patience throughout my study. Ellen Quataert Delft, January 2015 Wave runup on atoll reefs iii iv Contents Abstract i Acknowledgements iii Contents v 1 Introduction 1 1.1 Background 1 1.1.1 Hydrodynamics on reefs 2 1.1.2 Runup 3 1.1.3 Terminology 5 1.2 Scope and objectives of this study 6 1.3 Thesis outline 7 2 Kwajalein Atoll data analysis 9 2.1 Introduction 9 2.2 Study site and instrument deployment 10 2.3 Hydrodynamics across the reef 12 2.3.1 Methodology 12 2.3.2 Offshore wave conditions 14 2.3.3 Split frequency selection 15 2.3.4 Spectral evolution over the reef 16 2.3.5 Water level variations over the reef 19 2.4 Wave runup 22 2.4.1 Methodology 22 2.4.2 Wave runup observations 23 2.5 Discussion 24 2.6 Conclusions 25 3 The XBeach model 27 3.1 Introduction 27 3.2 XBeach model description 28 3.2.1 Hydrostatic model 28 3.2.2 Non-hydrostatic model 29 3.3 XBeach model applicability 30 4 Model results 33 4.1 Introduction 33 4.2 Model setup 34 4.3 Hydrodynamics across the reef 35 4.3.1 Calibration 36 4.3.2 Validation 42 4.4 Infragravity wave runup 44 4.5 Incident wave runup 46 4.5.1 Re-evaluation of the friction coefficients 46 4.5.2 Validation 50 4.6 Discussion 52 4.7 Conclusions 54 Wave runup on atoll reefs v 5 Conceptual model for wave runup on atoll reefs 55 5.1 Introduction 55 5.2 Atoll reef parameter space 56 5.3 Methodology 57 5.3.1 Conceptual model setup 57 5.3.2 Calculation of wave runup statistics 59 5.4 Conceptual model results 60 5.4.1 Infragravity wave runup 60 5.4.2 Incident wave runup 63 5.5 Conclusions 65 6 Conclusions and recommendations 66 6.1 Conclusions 66 6.2 Recommendations 68 References 70 Appendices A Additional XBeach hydrostatic results A-1 vi 1 Introduction 1.1 Background Coral reefs are capable of protecting mainland and island coasts from the impacts of severe storm waves and storm surges, the impact of tsunamis, and help limit coastal erosion (e.g.
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